A display device such as a liquid crystal display (LCD) and an electroluminescence (EL) display has been advanced in size and definition in recent years. Furthermore, high integration of circuits is also advanced by forming a pixel portion and peripheral circuitry for controlling the pixel portion integrally on a substrate.
In the case where element breakdown occurs due to patterning defects, electrostatic discharge (ESD) event and the like in the manufacturing process, a normal operation of the display device as a whole cannot be obtained, therefore, they are required to be eliminated by a quality inspection. Generally, a quality inspection of a display device is carried out in the phase of a module 1200 as a completed product as shown in FIG. 12A in which a TFT substrate 1201 over which a source driver 1203, a gate driver 1204, a pixel region 1205, a signal input terminal 1206 and the like are formed and a counter substrate 1202 are attached, and by inputting signals by actually using a jig 1211 and the like in order to display a picture or an image (a test pattern 1212 and the like) and observe the presence or absence of display defects by looking at the display as shown in FIG. 12B.
According to this method, however, an inspection is carried out in the phase that a display device itself is almost completed as the module 1200, therefore, there is a defect that much cost is spent on a module which is determined to be defective. That is, since a defect due to a circuit defect originates in the TFT substrate 1201 only, the process for attaching the counter substrate 1202 and the like results in waste. Further, in the case of manufacturing only a substrate (TFT substrate) whose pixel portion or peripheral circuitry is formed of a TFT and the like and shipping it as a semi-finished product, it is virtually impossible to carry out a quality inspection by an actual display in this case. That is, what is required is a means for determining a circuit operation to be normal or not in the state of a TFT substrate.
FIG. 11 is an example of a configuration which enables such an inspection. Formed over a substrate are a shift register (SR) and a NAND circuit 19, a data latch 20, a D/A converter (DAC) 21, a video data line 23, a digital source driver 18 formed of input terminals 22, 16 and the like of a signal, a power supply and the like, a gate driver 5, a pixel region in which pixels 3 are arranged in matrix, a capacitor line 15, a switch driver circuit 30, an analog switch 25, an inspecting line 27, an inspecting terminal 28 and the like.
In a display device shown in FIG. 11, each gate signal line 6 controls a pixel connected to a relevant line, and a video signal is inputted to a digital source driver 18, outputted to a source signal line 9, and written to each pixel.
In an inspecting circuit, a charge held in a capacitor 2 is taken out at an inspecting terminal 28 through the inspecting line 27 sequentially by writing video signals in a pixel through each pixel TFT 1, whereby writing to a pixel is determined to be good or defective. Further, the analog switch 25 is controlled by the switch driver circuit 30 (refer to Patent Document 1). There is a method for inspecting an output by touching a probe to each pad which is provided in each source line 9 for inspection (refer to Patent Document 2).
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-116423
[Patent Document 2]
Patent No. 2618042 Specification